Sorption and diffusion measurements have been made of methane, ethane and propane in the silica-alumina cracking catalyst previously employed in similar measurements for permanent gases (Barrer & Gabor 1959). The sorption data show no evidence of molecular sieve behaviour in the micropore structure of the catalyst, for the species studied. The ratio of surface to volume flow increases with increasing molecular weight of hydrocarbons and with decreasing porosity. On the other hand, in or near the Henry's law region of sorption, this ratio decreases as the temperature rises. In a micropore system there are many points where surface flow may be interrupted by roughnesses, blind pores or crevices. A treatment has been given which permits the influence of such pore properties upon surface diffusion to be considered. The procedure developed has been applied to the data obtained in this research, and estimates have been made of some properties of the surface diffusion coefficients which could be expected upon uninterrupted surfaces of the catalyst. Owing to differences in the roles of pores in steady and transient states of flow, transient state (time-lag) diffusion coefficients could depend on time. Some consequences of this have also been examined.